nix-generate-from-cpan: fix core module detection

This makes the detection of core modules a bit more robust by checking
the module inclusion in a pure Perl interpreter. This ensures that any
extra path in the `nix-generate-from-cpan` script's `PERL5LIB` does not
affect the generated package expression.

maintainers/scripts/nix-generate-from-cpan.nix

···
 { stdenv, makeWrapper, perl, perlPackages }:
 
 stdenv.mkDerivation {
-  name = "nix-generate-from-cpan-2";
+  name = "nix-generate-from-cpan-3";
 
   buildInputs = with perlPackages; [
     makeWrapper perl CPANMeta GetoptLongDescriptive CPANPLUS Readonly Log4Perl
···
   meta = {
     maintainers = with stdenv.lib.maintainers; [ eelco rycee ];
     description = "Utility to generate a Nix expression for a Perl package from CPAN";
+    platforms = stdenv.lib.platforms.unix;
   };
 }

maintainers/scripts/nix-generate-from-cpan.pl

···
     foreach my $n ( $deps->required_modules ) {
         next if $n eq "perl";
 
-        # Hacky way to figure out if this module is part of Perl.
-        if ( $n !~ /^JSON/ && $n !~ /^YAML/ && $n !~ /^Module::Pluggable/  && $n !~ /^if$/ ) {
-            eval "use $n;";
-            if ( !$@ ) {
-                DEBUG("skipping Perl-builtin module $n");
-                next;
-            }
+        # Figure out whether the module is a core module by attempting
+        # to `use` the module in a pure Perl interpreter and checking
+        # whether it succeeded. Note, $^X is a magic variable holding
+        # the path to the running Perl interpreter.
+        if ( system("env -i $^X -M$n -e1 >/dev/null 2>&1") == 0 ) {
+            DEBUG("skipping Perl-builtin module $n");
+            next;
         }
 
         my $pkg = module_to_pkg( $cb, $n );